A photo bomber is someone who either intentionally or unintentionally ruins an otherwise normal photo.
The "photo bomber" will be doing such things as: making faces, gestures, naked, or getting naked, in a costume, or doing some other equally hilarious action.

The photo bomber will usually work in the background of the photo. Some of the more ballsy photo bombers will go as far as being within inches behind or even next to the people in the picture , or in rare cases, run in FRONT of the photo.

The most ideal way to photo bomb is to successfully ruin their photo without letting the people know that you are forever ruining what could have been an amazing photo and memory, except you're sitting on a bench 15 yards behind them while you purposely have your cock and balls hanging out the top of your shorts while you eat an ice cream cone seductively.

Mom: Hey kids! I got the photos back from when we flew 1500 miles to Disney Land for our vacation!

Kids: Oh here's the picture of where we got to take a picture of all the Disney characters with us! This is our favorite picture ever!!!

Mom: Why is that man standing behind Goofy with his pants down and holding his penis with one hand pretending to ram Goofy in the ass with it?

And then the kids are forever traumatized. They will never see Disney character the same way again. And Disney Land will forever be tainted with visions and repressed memories that wont surface till they're in their 30's when their own kids say " I want to go to Disney Land!!!". And then you will murder you family after you go insane from repressed memories, go to prison where you will die of AIDS in your late 40's.

This would be the successful work of a "photo bomber" and/or a "photo bomb".